1. Field of the Invention.
The present invention relates to crystal growth, and, more particularly, to epitaxial growth of semiconductors.
2. Description of the Related Art.
Epitaxial growth of electronic grade semiconductors such as gallium arsenide (GaAs) relies on methods such as liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), and metalorganic chemical vapor deposition (MOCVD). MBE and MOCVD both provide the ability to grow extremely abrupt p-n junctions and heterojunctions of lattice-matched materials; and such abrupt junctions are required for fabrication of superlattices and quantum well structures. However, both MBE and MOCVD have serious shortcomings, and recently chemical beam epitaxy (CBE) has been proposed as a system to overcome these shortcomings by combining features of MBE and MOCVD; see, W. Tsang, Chemical Beam Epitaxy of InP and GaAs. 45 Appl. Phys. Lett. 1234 (1984).
The CBE system of Tsang (FIG. 1 is a schematic illustration) has the basic structure of an MBE system: a hemispheral vacuum chamber with sources arranged on the curved surface and aimed at the wafer holder located at the chamber center. The sources in MBE systems are effusion cells containing solid or molten elements (for example, growth of layers of Al.sub.x Ga.sub.1-x As with various x values and with silicon for n doping requires a cell for each of aluminum, gallium, arsenic, and silicon). Good uniformity across a wafer for MBE grown layers despite the nonuniformity of the elemental fluxes from the effusion cells can be achieved by rotation of the wafer during growth. A wafer is heated typically to 500.degree. to 700.degree. C. for growth to insure sufficient surface mobility of the deposited atoms so they can easily migrate to the appropriate lattice sites. In short, the growth conditions require simultaneous ultrahigh high vacuum, mechanical rotation, and heat for the wafers. These conditions pose a reliability problem for the wafer holders. The CBE system replaces some or all of the conventional elemental effusion cells with cells that are outlets of tanks of metalorganic gasses (for example, trimethylaluminum (TMAl), triethylgallium (TEGa), and trimethylarsine (TMAs)). This substitution solves the MBE problems of effusion cell life and flux drift, and the CBE cells are much simpler than the MBE effusion cells; but the problems of simultaneous high vacuum, mechanical rotation, and heat still remain.
The limited angle of flux produced by an MBE effusion cell or a CBE cell makes multiwafer operation very difficult. Movement of wafers past a stationary bank of MBE or CBE cells is not practical due to the complexity of such an arrangement (and the flux drift of MBE cells). Thus a problem of small throughput exists for both MBE and CBE.